1. Technical Field
The present invention relates to servo control to reduce track mis-registration (TMR). More specifically, the present invention applies to enhanced disturbance rejection when a shock or vibration occurs to correct for effects of that disturbance.
2. Related Art
A hard disk drive typically includes one or more rotatable storage media, or disks upon which data is encoded. The disks are mounted on the shaft of a spindle motor for rotation. Data is encoded on the rotating disks as bits of information using magnetic field reversals grouped in tracks. A transducer head supported by an actuator arm is used to read data from or write data to the disks.
A voice control motor (VCM) attached to the actuator arm controls positioning of the actuator, and thus the transducer head position over a disk. Current is applied to the coil of the VCM to control the position of the actuator. Movement of the actuator caused by current applied to the VCM, or by an external shock, generates a back emf voltage in the coil of the VCM motor. Measurements of back emf from the VCM coil are typically made to determine the velocity of the actuator during start-up, or until track positioning information can be read from the disk through the transducer head to determine actuator position.
The transducer head includes a slider having an air bearing surface that causes the transducer to fly above the data tracks of the disk surface due to fluid currents caused by the spindle motor rotating the disk. Thus, the transducer does not physically contact the disk surface during normal operation of the disk drive to minimize wear on both the head and disk surface.
Typically during shut down, the actuator is positioned on a ramp situated off to the side of a disk. For some disk drives, the ramp may be at or near the inner diameter of the disk. Before power is actually shut off, the actuator assembly is moved up the ramp to a park position at the top of the ramp to prevent the slider from contacting the disk.
Startup includes moving the actuator down the ramp so that the slider of the transducer flies when it gets to the bottom of the ramp. To assure the slider does not come into contact with the disk, the velocity of the actuator coming down the ramp is carefully monitored and controlled. Since servo data cannot be read using the transducer head, back emf voltage across the VCM coil is measured to monitor actuator velocity since back emf varies as a function of the velocity of the actuator sliding down the ramp.
Once the slider forms an air-bearing over the disk, the head can typically read from the disk. Servo position data read from the disk is processed by the processor, enabling the processor to provide servo control signals to control the VCM for proper positioning of a transducer head relative to a disk. With servo position data available for determining actuator position, back emf voltage readings in previous systems in most disk drive systems are no longer used to determine the position and/or velocity of the actuator.
Control of the position of the head over tracks on the disk is typically achieved with the closed loop servo system where head position servo information is provided from the disk to detect and control the position of the heads. As will be recognized, a dedicated servo system entails the dedication of one entire surface of one of multiple disks to servo information, with the remaining disc surfaces being used for the storage of user data. Alternatively, an embedded servo system involves interleaving the servo information with the user data on each of the surfaces of the discs so that both servo information and user data is read by each of the heads. Hybrid systems also exist, in which one entire disk surface is dedicated to servo information and a small portion of each remaining disk surface also contains servo information.
Servo data read enables measurement or estimation of various parameters including head position, velocity and acceleration and to use these parameters in the closed-loop control of the position of the head. For example, during track following where a head is controlled to follow a selected track on the disk, track mis-registration (TMR) is determined using a position error signal (PES) generated from servo information on the disk to indicate relative distance between the head and the center of the selected track. The PES is used to generate correction signals which adjust the position of the head by adjusting the amount of current applied to the VCM coil. Additionally, during a seek, which involves the acceleration and subsequent deceleration of the head from an initial track to a destination track on a disk, the measured or estimated radial velocity of the head can be compared to a model or profile velocity, with correction signals being generated from the differences between the actual velocity and the profile velocity of the head.
Besides servo data read from a disk and back emf of the VCM coil, the amount of movement of the actuator due to a shock or vibration can be predicted using other components in the disk drive. As one example, the back emf of the spindle motor may be used. Back emf from winding coils of the spindle motor is typically monitored during operation of the disk drive to assure the spindle motor is operating at a desired speed. An external shock applied to the disk drive will cause a sudden measurable change in the spindle motor speed. Spindle rotational speed can also be monitored by observation of the time between servo samples read from the disk. As another example, one or more rotational and/or linear accelerometers can be included in the disk drive for the purpose of measuring external shocks or vibrations applied. Accelerometers are more typically used in notebook or more mobile computers where shocks or vibrations may be expected during operation to enable corrections to be made should the actuator be knocked out of position, or at least to allow a write operation to be halted before any damage is done to tracks adjacent to the target track.
With increased track densities and rotational speeds of disk drives, closed-loop control of head position has become increasingly critical to minimize TMR. In one case to improve control of actuator head position, combining measurement techniques to determine actuator movement has been contemplated. The combined measurement techniques included measurement of back emf from the VCM which was continued after servo data could be read from the disk. This combination of measuring back emf and using servo data to correct for TMR is disclosed in U.S. Pat. No. 5,844,743, entitled “Velocity Sensing Using Actuator Coil Back-EMF Voltage,” which is incorporated herein by reference.
Combining measurement techniques to measure and correct for actuator movement caused by vibrations or shocks, however, may not improve the performance of a system controlling TMR. The additional sensors used may provide noisy or low-resolution signals, and servo corrections made on the basis of those signals may actually add more TMR than they eliminate. To improve disk drive system performance, there is a continuing need for improved approaches to control TMR.